The present invention relates generally to the field of cyclotron design for radiopharmacy and more particularly to a method and apparatus that can improve ion source lifetime and performance.
Hospitals and other health care providers rely extensively on positron emission tomography (PET) for diagnostic purposes. PET scanners can produce images which illustrate various biological process and functions. In a PET scan, the patient is initially injected with a radioactive substance known as a PET isotope (or radiopharmaceutical). The PET isotope may be 18F-fluoro-2-deoxyglucose (FDG), for example, a type of sugar which includes radioactive fluorine. The PET isotope becomes involved in certain bodily processes and functions, and its radioactive nature enables the PET scanner to produce an image which illuminates those functions and processes. For example, when FDG is injected, it may be metabolized by cancer cells, allowing the PET scanner to create an image illuminating the cancerous region.
PET isotopes are mainly produced with cyclotrons, a type of particle accelerators. A cyclotron usually operates at high vacuum (e.g., 10−7 Torr). In operation, charged particles (i.e., ions) are initially extracted from an ion source. Then, the ions are accelerated while being confined by a magnetic field to a circular path. A radio frequency (RF) high voltage source rapidly alternates the polarity of an electrical field inside the cyclotron chamber, causing the ions to follow a spiral course as they acquire more kinetic energy. Once the ions have gained their final energy, they are directed to a target material to transform it into one or more desired PET isotopes. Since a cyclotron typically involves a substantial investment, its isotope-producing capacity is very important. Theoretically, the production rate of isotopes in a given target material is directly proportional to the flux of the charged particles (i.e., ion beam current) that bombard the target. Therefore, it would be desirable to extract a high output of ion current from the ion source.
Apart from the ion output, the lifetime of an ion source is also important. An ion source typically has a limited lifetime and therefore requires periodic replacement. During a scheduled service, the cyclotron needs to be opened up to allow access to the ion source. However, since the cyclotron usually becomes radioactive during isotope production, it is necessary to wait for the radiation to decay to a safe level before starting the service. In one cyclotron, for example, the wait for the radiation decay can last ten hours. Replacement of the ion source takes some time depending on the complexity of the ion source assembly as well as its accessibility. After the ion source has been replaced, it takes additional time for a high vacuum to be restored inside the cyclotron. As a result, every scheduled service for ion source replacement causes extended down time in isotope production. Therefore, it would be desirable to improve the lifetime of the ion source so that the isotope production time will be longer between scheduled services.
FIG. 1 illustrates the operation of a known plasma-based ion source 100 used in cyclotrons for isotope production. As shown, the ion source 100 comprises an ion source tube 104 positioned between two cathodes 102. The ion source tube 104 may be grounded while the two cathodes 102 may be biased at a high negative potential with a power source 112. The ion source tube 104 may have a cavity 108 into which one or more gas ingredients may be flowed. For example, a hydrogen (H2) gas flow of around 10 sccm may be flowed into the cavity 108. The voltage difference between the cathodes 102 and the ion source tube 104 may cause a plasma discharge (110) in the hydrogen gas, creating positive hydrogen ions (protons) and negative hydrogen ions (H−). These hydrogen ions may be confined by a magnetic field 120 imposed along the length of the ion source tube 104. A puller 116, biased with a power source 114 at an alternating potential, may then extract the negative hydrogen ions through a slit opening 106 on the ion source tube 104 during positive half periods of the alternating potential. The extracted negative hydrogen ions 118 may be further accelerated in the cyclotron (not shown) before being used in isotope production.
FIGS. 2–7 illustrate a prior art design of an ion source tube 200, where FIG. 2 is a perspective view of the ion source tube 200, FIG. 3 is a front view, FIG. 4 is a side view, FIGS. 5 and 7 are cross-sectional views of the section a—a, and FIG. 6 is a cross-sectional view of the section b—b. The length unit is millimeters (mm). The ion source tube 200 has a cylindrical cavity 212 that is centered along the axis 216. There is also a slit opening 214 along the front side of the ion source tube 200. This prior art design further requires two separate restrictor rings 210 that can be inserted into the cavity 212 and positioned against the edges 220 and 222 to help define the shape and position of the plasma column 218.
Some drawbacks may exist in the design of the prior art ion source tube 200. For example, the use of the restrictor rings 210 may require some amount of time for assembly and adjustment during manufacturing. And the prior art design of the restrictor rings may impose a stringent manufacturing tolerance. Furthermore, the slit opening 214 can degrade relatively quickly due to bombardment of the ions generated in the plasma column 216, leading to a short lifetime of the ion source tube 200.
These and other drawbacks may exist in known systems and methods.